1. Field of the Invention
The present invention relates in general to circuits for diverting electrical power, and in particular to power diverting circuits for creating a supply voltage for a signal processing circuit from a source of data signals having a varying potential.
2. Description of Related Art
A computer may be connected to a peripheral device, such as a printer or external drive, by a cable or bus containing several signal lines running in parallel between the computer and the peripheral device. In order to control the operation of the peripheral device, the computer sends data signals thereto via the cable. It is often desirable, however, to be able to perform additional processing of these data signals by means of a separate circuit connected to the cable which receives and processes the data signals sent to the peripheral device.
In some applications, it is inconvenient to power such a signal processing circuit by a separate power supply. Such applications are provided with a power diverting circuit which diverts some power from the data signals received by the signal processing circuit and provides that power at an output terminal to a power supply terminal of the signal processing circuit.
FIG. 1 shows a block diagram of a computer 1 and a peripheral device 2 which are connected together by a data bus 3. Data signals are sent along the data bus 3 by means of several data signal lines, indicated by the references 3.1, 3.2, . . . , 3.n. A signal processing circuit 4 is also coupled to the data bus 3 by means of its inputs 4.1, 4.2, 4.n, respectively connected to the data signal lines 3.1, 3.2, . . . , 3.n. A power diverging circuit 5 is connected to the inputs of the signal processing circuit 4 and diverts some power from the data signals received at these inputs to a first power supply terminal 6 of the signal processing circuit 4.
The signal processing circuit 4 is also provided with a second power supply terminal 7 connected to a ground supply 8, this latter being connected to the ground supply (not shown) of either the computer 1 and/or the peripheral device 2. The voltage between the power supply terminals 6 and 7 drives the signal processing circuit 4. A capacitor 9 is connected between the power supply terminal 6 and the ground supply 8 in order to smooth the voltage supplied to the signal processing circuit 4 between the power supply terminals 6 and 7.
The power diverting circuit shown in FIG. 1 comprises a series of diodes or equivalent circuits, referenced 5.1, 5.2, . . . , 5.n, having their anodes respectively connected to the signal processing circuit inputs 4.1, 4.2, . . . , 4.n. The cathodes of the diodes 5.1, 5.2, . . . , 5.n are connected together to the power supply terminal 6. It is often convenient to realise the signal processing circuit 4 and the power diverting circuit 5 in the form of an integrated circuit 10. The smoothing capacitor 9 is generally external as it is usually too large to be included in the integrated circuit 10.
In the case of TTL-compatible logic being used to communicate between the computer 1 and the peripheral 2, the data signals will either have a high potential of between 2.4 and 5.5 volus or a low potential of less than 0.8 volts, depending respectively on whether the data signal represents a logical "high" or a logical "low". In the event that the potential at the anode of one or more of the diodes 5.1, 5.2, . . . , 5.n exceeds the potential at the power supply terminal 6 by an amount greater than the forward-biased voltage-drop across that diode, a bias current I.sub.bias will be caused to flow through that diode to the power supply terminal 6 so as to drive the integrated circuit 10.
The power diverting circuit 5 operates correctly provided that most of the time at least some of the data signal lines 3.1, 3.2, . . . , 3.n are at a logical "high" and provided that these logically "high" lines can provide the necessary bias current I.sub.bias to drive the signal processing circuit 4. In order to assure that a sufficient bias voltage V.sub.DD is provided between the power supply terminals 6 and 7 for critical circuit functions, such as those provided by the analog blocks of the signal processing circuit 4, the voltage-drop across the power diverting circuit 5 must be minimised.
U.S. Pat. No. 5,218,235 describes a power diverting circuit which obtains a low voltage-drop in conduction by the use of a MOS transistor switch and a comparator associated with each input of the signal processing circuit 4. FIG. 2 represents part of the power diverting circuit described in the above-mentioned patent and shows a comparator 20 and a MOS transistor switch 21 associated with the input 4.1 of signal processing circuit 4. The comparator 20 has a positive input connected to the power supply terminal 6 and a negative input connected to the input 4.1 of the signal processing circuit 4. The comparator 20 is also connected to the power supply terminal 6 and the ground supply 8 and is driven by the voltage V.sub.DD there between.
A p-type MOS transistor switch 21 has its drain and source connected respectively to the power supply terminal 6 and the data processing circuit input 4.1. The gate of the MOS transistor 21 is controlled by the output of the comparator 20 in such a way that the MOS transistor 21 is caused to conduct when the voltage V.sub.IN between the input 4.1 and the ground supply is greater than the voltage VDD. Conversely, the MOS transistor 21 is caused to be non-conducting when the voltage V.sub.IN is less than the voltage VDD.
However, this solution provides a number of inconveniences. Firstly, the described power diverting circuit is quite complex, requiring the use of a separate comparator for each data signal line. The realisation of a large number of comparators, in an integrated form, requires the use of many transistors which thus occupy a large area and involve a significant power consumption. Secondly, by causing each MOS transistor to conduct only when the voltage V.sub.IN is greater than the voltage V.sub.DD, the power supply voltage V.sub.DD provided by the power diverting circuit is sometimes unstable and ill-suited for use with signal processing circuitry. Thirdly, the implementation of logic decisions, such as the activation or deactivation of the power diverting circuit, may not be easily achieved.
An object of the present invention is to provide an efficient power diverting circuit for creating a supply voltage for a signal processing circuit from a source of data signals having a varying potential which overcomes or ameliorates the problems of existing power diverting circuits.